1. Field of the Invention
The present invention relates to a calibration method to calibrate a substrate table position in a lithographic apparatus. Further, the invention relates to a lithographic apparatus including a control system to control a position of a substrate table, the control system being arranged to calibrate the position of the substrate table. Also, the present invention relates to a patterning device for a lithographic apparatus.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In the lithographic apparatus, a position of the substrate table is measured by a substrate table position measurement system including one or more position sensors. The position sensors may include e.g. an interferometer and/or an encoder. The interferometer is a type of optical measurement device which measures a distance towards a reference, commonly a mirror or other reflecting surface. The encoder is a type of optical measurement device which measures a position of a reference by detecting an interaction of an optical beam of the interferometer with a pattern on the reference, the pattern including e.g. a grid, a grating, etc. Depending on a position of the reference, a different reflection, diffraction, etc. (depending on a type of interferometer) will occur, which is detected, thereby providing position information regarding the reference. In existing designs of lithographic apparatuses, use may be made of a substrate table which includes reflecting sides. The sides are used as mirrors by interferometers which direct a measurement beam to the reflecting sides. A plurality of interferometers may be used, each directed at e.g. a different side of the substrate table, or two or more interferometers being directed to a same side of the substrate table, thereby providing information as to a length of an optical part between the interferometer and the reflecting side of the substrate table. Thereby, a position of the substrate table may be measured in a plane substantially parallel to the surface of the substrate held by the substrate table. This plane is commonly indicated by an X, Y plane, while a dimension perpendicular to the surface of a substrate held by the substrate table is commonly referred to as a Z dimension. By such a combination of interferometers, a position of the substrate table may be provided in X direction, Y direction, as well as in a rotation with respect to the Z axis.
However, the references, thus in this example the reflecting sides of the substrate table, may show imperfections, in particular may show irregularities on the surface thereof. As the interferometers measure a length of an optical path, any unflatness of the reflecting sides will result in an error in the measurement of the position of the substrate table. Assume for example that interferometers are directed towards two sides of the substrate table, one side being substantially parallel to the X direction, while the other side being substantially parallel to the Y direction. The interferometers directed to the side of the substrate table parallel to the X direction provide information as to the position of the substrate table in the Y direction, and vice versa. Further, by having two or more interferometers directed to a same side of the substrate table information as to a rotation of the substrate table with respect to the Z axis, may be provided. An irregularity in a flatness of the mirror (thus the reflecting side) parallel to the X axis will translate into a measurement error in the position of the substrate table in Y direction, while an unflatness at the side of the substrate table parallel to the Y axis will translate into a measurement error of the position of the substrate table in X direction. By its nature, a deviation of the surface of the side of the substrate table against which the interferometer beam reflects, may differ depending on the position at which the beam reflects at that surface.
Therefore, a position dependent error has come into existence, as a deviation of the reflecting surface may differ for each part of that surface. As a practical approximation, an error of the interferometer to measure a position of the substrate table in X direction is dependent on a position in Y direction of the substrate table, and vice versa. Further, an error in a measurement of a rotation of the substrate table with respect to the Z axis is dependent on a position of the substrate table in Y direction, assuming that the rotation is measured by directing two or more interferometers towards a side of the substrate table which is parallel to the Y direction.
A calibration may now be performed by repeatedly irradiating a pattern onto the surface of the substrate, the substrate being displaced between successive irradiations thereby irradiating the patterns next to each other, or partly overlapping, to form a single dimensional arrangement of patterns on the surface of the substrate, the arrangement extending preferably in X direction or Y direction. The patterns are read out and incremental position deviations are derived from reading out neighbouring (e.g. partly overlapping) patterns. Therefrom a position error is derived which may be used to calibrate the position of the substrate table in the dimension in question. It is noted that this calibration does not provide a separate calibration for the interferometer, however provides a calibrations of the positioning system as a whole which provides for the positioning of the substrate table, the interferometer in question forming part of that positioning system.
In recent designs of lithographic apparatuses, requirements as to an accuracy of a position measurement of the substrate table are increased. To be able to provide a sufficiently accurate position measurement of the substrate table, use has been made in such configurations of a different type of position measurement, as compared to the interferometer configuration described above. In this configuration, use is made of a two dimensional grating provided over the substrate table. The substrate table is provided with a plurality of sensors which provide position information by directing appropriate measurement beams towards the grid or grating. The measurement sensors may e.g. include interferometers, encoders, or any combination thereof, depending on the position information to be obtained from that particular sensor. In these configurations, each of the sensors is prone to an error which is dependent on the position of the substrate table in X direction as well as the position of the substrate table in Y direction. From the signals provided by the individual sensors, a position of the substrate table in up to 6 degrees of freedom may be derived. The position of the substrate table in each of these degrees of freedom may thus show an error which is dependent on the position of the substrate table in X direction as well as in Y direction. To be able to calibrate the position of the substrate table in such a configuration, the calibration as described above does not provide sufficient results, as it does not take into account the multidimensional source of error which comes forward here.